This invention is directed to particle counting methods and apparatuses which provide a time correction to the time period particle pulses are counted so that coincidence phenomena resulting in fewer particle pulses being counted in a predetermined time period does not induce an error in the particle pulse count ultimately obtained. The particle counting methods and apparatuses concerned employ particle sensing zones wherein the presence of one particle within the zone masks or hides the presence of another particle which also may be within the sensing zone. This invention particularly is directed to, but not limited to, the correct determination of non-electrical properties, such as size and count of microscopic particles by measuring electrical properties (Patent Office Class 324-71NE).
Now well known in the art of electronic particle counting and analyzing is apparatus marketed primarily under the trademark "Coulter Counter," and owned by the assignee of this application. Such apparatus and portions thereof are disclosed in several U.S. patents, for example U.S. Pat. Nos. 2,656,508; 2,985,830; and 3,259,842 (class 324-71). A significantly important portion of such Coulter type of apparatus is the minute scanning aperture or scanning ambit or sensing zone relative to or through which pass and are detected and counted, single particles at a rate often well in excess of one thousand per second. Because of the physical parameters of the scanning aperture, and particle concentration, coincidence of two particles in the scanning ambit occurs quite often.
It has been found that once a particle pulse produced by the presence of a particle in the scanning ambit, exceeds the threshold setting of the apparatus and is detected, no other pulse will be produced in response to other particles in the scanning ambit until such time as the particles have cleared the sensing zone. As the presence of a first particle masks a second particle causing a coincidence error, the coincidence error is a function of the time that detected particles are in the scanning ambit, and the total coincidence error is a function of the total time that particles are in the scanning aperture during the detecting and counting process. Since the coincidence error is a function of the time that each particle is in the scanning aperture, it can also be said that the coincidence error is a function of the duration of each particle pulse produced and detected in response to passage of a particle through the scanning ambit. It has also been found that the coincidence error is functionally related to the rate at which particles pass through the scanning ambit and, therefore, the particle pulse repetition rate if this rate is determined only by particle concentration.